Conventionally, a small speaker device has a difficulty in realizing bass reproduction due to a small cabinet volume. This is because when a cabinet volume is small, the effect of an acoustic stiffness of a chamber in a cabinet is large. As a mean of easily realizing bass reproduction in the small speaker device, a speaker device, in which an activated carbon is provided in the cabinet, has been proposed (see patent document 1, for example).
FIG. 14 is a cross-sectional view illustrating a main portion of a conventional speaker device. In FIG. 14, the conventional speaker device includes a cabinet 1, a woofer 2, an activated carbon 3, a supporting member 4, a diaphragm 5, and an air tube 6. The woofer 2 is attached to the front of the cabinet 1. The activated carbon 3 is disposed in the cabinet 1 in the form of a mass or lump. The activated carbon 3 is supported by a back face, a bottom face, an upper face, and left and right side faces of the cabinet 1, as well as the supporting member 4. Small air holes for passing air are formed on an entire surface of the supporting member 4. The air tube 6 is provided to the diaphragm 5. The air tube 6 passes air between the activated carbon 3 and the woofer 2.
Described next is an operation of the aforementioned speaker device. When an electrical signal is applied to the woofer 2, a sound pressure is generated. A pressure in the cabinet 1 is changed by the sound pressure. Then, the diaphragm 5 is vibrated by the pressure which has been changed. By the vibration of the diaphragm 5, a pressure in a chamber having the activated carbon is changed. The activated carbon 3, provided in the form of a mass or lump, is supported by the supporting member 4 and the cabinet 1, and the small air holes are provided on the entire surface of the supporting member 4. Therefore, gas affected by the pressure change caused by the vibration of the diaphragm 5 is physically adsorbed into the activated carbon 3, thereby suppressing the pressure change in the cabinet 1. Note that the air tube 6 is provided so as to suppress a pressure change in a space, including the activated carbon 3, enclosed by the cabinet 1 and the diaphragm 5, the pressure change caused by changes in ambient temperature or air pressure of the speaker device.
As described above, in the conventional speaker device, the cabinet 1 equivalently operates as a larger volume cabinet. Thus, the conventional speaker device having a small cabinet is able to realize bass reproduction as if the speaker unit is provided in a large cabinet.
[Patent document 1 ] Japanese Unexamined Patent Publication No. 60-500645
The activated carbon 3 has pores, each of which has a size in the order of micrometers, formed thereon. With the pores, the activated carbon 3 physically adsorbs gas. However, when moisture, organic gas (ammonia, for example) and the like are adsorbed into the activated carbon 3, the pores formed on the activated carbon 3 are clogged by the moisture or organic gas, thereby deteriorating the effect of physical adsorption provided by the activated carbon 3. Thus, there is a problem in that the activated carbon 3 adsorbs gas such as moisture or organic gas, which causes deterioration of the activated carbon 3, thereby reducing the effect of the activated carbon 3 provided for suppressing a pressure change in the cabinet 1.
In the speaker device disclosed in the aforementioned patent document 1, the activated carbon is also provided in the air tube 6 so as to suppress deterioration of the activated carbon 3 enclosed by the cabinet 1. Specifically, because the activated carbon provided in the air tube 6 deteriorates faster than the activated carbon 3, the progression of deterioration of the activated carbon 3 can be delayed. However, once the activated carbon provided in the air tube 6 is deteriorated, deterioration of the activated carbon 3 proceeds in a similar manner to the case where no activated carbon is provided in the air tube 6. That is, in the speaker device disclosed in the aforementioned patent document 1, the progression of deterioration of the activated carbon 3 can be delayed only until the activated carbon provided in the air tube 6 is deteriorated. Thus, in the conventional speaker device, it is difficult to maintain, over a long period of time, the effect of the activated carbon 3 provided for suppressing a pressure change in the cabinet 1.